RFR: 8157490: null stream->source() no longer causes error with -Xlog:class+load

[Max Ockner]

Hello,

Please review this very small fix for class+load logging with modules.

Bug: https://bugs.openjdk.java.net/browse/JDK-8157490
Webrev: http://cr.openjdk.java.net/~mockner/8157490/

Summary:  The JCK test 
vm/jni/DefineClass/dfcl001/dfcl00101m1/dfcl00101m1 hit a SIGSEGV in the 
logging section at the end of ClassFileParser::fill_instance_klass(). 
With the addition of modules, this logging section now uses

     strlen(stream->source())

to compute some of the module related parameters that it needs. This 
SIGSEGV originated from the above line being executed with a null stream 
source.

Tested with vm/jni/DefineClass/dfcl001/dfcl00101m1/dfcl00101m1

Thanks,
Max